Some materials aren't rendered in the Blender Game Engine
I am having the same problem. Best solution I could find (more like a workarround) is to apply all the modifiers of the object which is causing trouble (in our case, it is Luftauslassheck).

Should you find out what is happening, please let me know. Here is a link to my blend if you want to take a look at it. The object is Support.Cover, which only has the material PlasticMaterial.002, used in other objects as well.

Nice car model by the way